Electrocomposite coatings for hard chrome replacement

ABSTRACT

The invention provides a method and system for electrolytically coating an article. The method includes providing an article to be coated and disposing the article in an electrolytic cell. The cell includes an anode, a cathode in operable communication with the article, and an electrolyte bath. During electrolysis, the electrolyte bath comprises cobalt ions, phosphorous acid, and tribological particles selected from the group consisting of refractory materials, solid lubricants and mixtures thereof dispersed therein. The method further includes applying steady direct electric current through the anode, the electrolyte bath and the cathode to coat the article with cobalt, phosphorous and the tribological particles. An improved composition of matter is also provided that may be used as a coating, or the composition may be electroformed on a mandrel to form an article made from the composition of matter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No.13/111,314, filed May 19, 2011, now U.S. Pat. No. 8,202,627, which inturn is a continuation-in-part of and claims the benefit of priority toU.S. patent application Ser. No. 12/331,623, filed Dec. 10, 2008, nowU.S. Pat. No. 8,168,056, which in turn is a continuation of and claimsthe benefit of priority to U.S. patent application Ser. No. 11/510,417,filed Aug. 26, 2006, which in turn claims the benefit of priority toU.S. Provisional Patent Application Ser. No. 60/761,445, filed Jan. 24,2006. This application also claims the benefit of priority to U.S.Provisional Patent Application Ser. No. 61/346,165, filed May 19, 2010.The disclosure of each of the aforementioned applications isincorporated by reference herein in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to improved methods and systems forcoating materials as well as improved protective coatings for materials.Particularly, the present disclosure is directed to methods and systemsfor making coatings including cobalt, phosphorous and particles ofmaterial having superior tribological characteristics.

2. Description of Related Art

Electroplated hard chrome coating is widely used as a wear resistantcoating to prolong the life of mechanical components. However,conventional hard chrome electroplating processes generate hexavalentchromium ion which is a known carcinogen. Hence, there is a major effortthroughout the electroplating industry to replace hard chrome coatingswith an environmentally benign, non-carcinogenic coating havingcharacteristics similar or superior to those of hard chrome.

Thermal spray hard coatings of chromium carbide, tungsten carbide,tribaloy, aluminum oxide and the like, using Plasma Spray, High VelocityOxy Fuel (HVOF) and other similar processes are currently being used toreplace hard chrome coatings. However, these processes have not beenable to be used for non line of sight (NLOS) applications, such as theinner diameter (ID) of cylinders, bearing cavities and the like. Evenfor the outer surface applications, thermal spray coatings are generallydeposited in thick layers and later ground to a desired thickness.Hence, thermal sprayed coatings are generally more expensive thanelectroplated hard chrome.

For NLOS applications, a number of electroplated coatings have beenevaluated. These include electroplated Ni—P and Ni—W alloy coatings,Ni—SiC electrocomposite and other similar coatings. However, none ofthese coatings have all the desired characteristics of hard chrome.Also, nickel base coatings are now considered undesirable because it hasbeen found that in some cases they can cause severe allergic reactions.

Recently, a new nanocrystalline Co13 P base coating has been developedby pulse plating processes. The resulting nanocrystalline Co13 P coatingappears to be a very promising replacement for hard chrome as itscharacteristics are either equal or superior to those of hard chrome.However, the electroplating process for this nanocrystalline Co13 P basecoating is based on pulse plating. In pulse plating, the applied voltagebetween the anode and cathode is pulsed at different amplitudes and atvarious frequencies. This pulse plating process used to producenanocrystalline Co13 P coatings requires special power supplies whichare currently available only for laboratory research and development.Large scale affordable pulsed power supplies for the productionenvironment are not currently available.

In another aspect, engineered components are typically coated to providewear resistance, corrosion resistance, and oxidation resistance of thebase material. The most widely used coating is hard chrome because ofits excellent wear resistance, low cost and ease of application. Hardchrome also has its own deficiencies, among them is adverse effects onthe fatigue strength of base materials. Because of this undesirableimpact on fatigue resistance or a “fatigue debit”, base materials areappropriately derated requiring heavier cross sections, weight and costpenalty. Fatigue debit results from micro cracks inherent in the hardchrome deposition that act as crack initiation sites. As a result,engineers have to design plated hardware susceptible to fatigueaccordingly. This usually requires larger, stronger cross sections tocompensate for the fatigue debit induced by the plating.

Hard chrome is not the only coating that creates a “fatigue debit.”Electroless nickel and High Velocity Oxy Fuel (HVOF thermal spray)coatings also have “fatigue debits”. High phosphorous electroless nickelhas an amorphous, glassy brittle micro-structure, which is susceptibleto cracking which causes a fatigue debit, while mid phosphorouselectroless nickel and HVOF coatings contain porosities in theircoatings. The not fully dense structure is similar to a crackedstructure resulting in fatigue debit. Table A demonstrates typicalfatigue debit data for electroless nickel and hard chromium.

TABLE A Typical “Fatigue Debit” results for Hard Chromium andElectroless Nickel Bar, Coating Type Avg. Fatigue Cycles Fatigue Debit,% Uncoated 4130* 117,426 (4 pcs) — Hard Chromium  34,370 (3 pcs) 70%(0.002″) Uncoated 4130*  75,185 (2 pcs) — Hard Chromium  27,468 (3 pcs)64% (0.002″) Uncoated 4130* 156,252 (3pcs) — Electroless Nickel  85,019(3pcs) 46% Uncoated 4130  75,185 (2 pcs) — Cr over Electroless  39,059(3 pcs) 48% Ni *Different manufactured lots of fatigue specimens givedifferent baselines for fatigue

One solution to counter the “fatigue debit” of plated coatings oftenused on fatigue sensitive hardware is the use of shot peening. Shotpeening imparts a compressive stress on the surface of the material tobe coated which retards the initiation of fatigue cracks. The need toshot peen hardware which will be electroplated adds expense to theoverall manufacturing process. Therefore, there exists a need forcoatings used on engineered stress-bearing components which provide wearand corrosion resistance without a “fatigue debit” of base materials.

As set forth above, there is a continued need for improved coatings andassociated processes for replacing hard chrome. The present disclosureprovides a solution for these and other problems.

SUMMARY OF THE DISCLOSURE

The purpose and advantages of the presently disclosed embodiments willbe set forth in and become apparent from the description that follows.Additional advantages of the disclosed embodiments will be realized andattained by the methods and systems particularly pointed out in thewritten description and claims hereof, as well as from the appendeddrawings.

As embodied herein, in one aspect, the disclosure includes a method forelectrolytically coating an article. The method includes providing anarticle to be coated and disposing the article in an electrolytic cell.The cell includes an anode, a cathode in operable communication with thearticle, and an electrolyte bath. During electrolysis, the electrolytebath comprises cobalt ions, phosphorous acid, and tribological particlesselected from the group consisting of refractory materials, solidlubricants and mixtures thereof, dispersed therein. The method furtherincludes applying steady direct electric current through the anode, theelectrolyte bath and the cathode to coat the article with cobalt,phosphorous and the tribological particles.

In accordance with a further aspect of the disclosure, the electrolytebath may include, for example, tribological particles of refractorymaterial selected from the group consisting of ceramics, diamond andmixtures thereof. In accordance with one aspect of the disclosure, theelectrolyte bath may include ceramic tribological particles selectedfrom the group consisting of silicon carbide, chromium carbide, boroncarbide, tungsten carbide, titanium carbide, silicon nitride, aluminumoxide, chromium oxide, and mixtures thereof. In accordance with anotheraspect of the disclosure, the electrolyte bath may include solidlubricant tribological particles selected from the group consisting ofgraphite, boron nitride, polytetrafluoroethylene (“PTFE”), molybdenumdisulfide, tungsten disulfide, and mixtures thereof.

In accordance with another aspect of the disclosure, the phosphorousacid may be present in the electrolyte bath in a concentration fromabout 3 grams per liter to about 35 grams per liter. In accordance withanother embodiment of the disclosure, the phosphorous acid is present inthe electrolyte bath in a concentration from about 3 grams per liter toabout 25 grams per liter. In accordance with a preferred embodiment ofthe disclosure, the phosphorous acid is present in the electrolyte bathin a concentration from about 3 grams per liter to about 15 grams perliter.

If desired, the anode may include a portion formed from consumablecobalt material adapted to release cobalt ions into the electrolyte bathas cobalt is deposited on an article to be coated. The consumable cobaltanode may comprise a cobalt plated electrode, and/or may include piecesof cobalt disposed in a basket or other suitable container incommunication with the electrolyte bath. The source of cobalt ions mayadditionally or alternatively include, for example, a soluble cobaltsalt selected from the group consisting of CoSO₄, CoC1₂, CoCO₃,Co(SO₃NH₂)₂ and mixtures thereof disposed in the electrolyte bath. Ifdesired, an inert anode may be provided formed from a material selectedfrom the group consisting of graphite, platinized copper, platinizedtitanium, platinized columbium or combinations thereof.

It is also possible to perform electroforming operations to producecobalt parts in accordance with the disclosure. In accordance with thisaspect of the disclosure, the cathode acts as a master, whereby asubstrate, or coating, may be formed on the cathode and then removedfrom the cathode as a separate piece. The cathode may accordingly bemade from a material that does not adhere significantly to the coatingto facilitate its removal, such as passivated stainless steel. Inaccordance with a further aspect of the disclosure, the article to becoated may be the cathode of the cell.

In accordance with another aspect of the disclosure, the tribologicalparticles in the electrolyte bath may have an average dimension betweenabout 0.1 micrometers and about 20 micrometers. In accordance with apreferred embodiment of the disclosure, the tribological particles mayhave an average dimension between about 1.0 micrometers and about 5.0micrometers.

In accordance with yet a further aspect of the disclosure, theelectrolyte bath may further comprise a dissolution promoter forpromoting the dissolution of the consumable cobalt material. Thedissolution promoter may include, for example, a metal halide salt. Inaccordance with certain specific embodiments of the disclosure, thedissolution promoter may be selected from the group consisting of sodiumchloride, cobalt chloride, metal bromide salts and combinations thereof.If desired, the electrolyte bath may further comprise a buffering agent,such as boric acid to help maintain the pH within a desired tolerance.Moreover, a pH adjustor may also be employed to control the pH of thesystem, such as cobalt carbonate, sodium hydroxide and sulfuric acid.

In accordance with one embodiment of the disclosure, the pH of theelectrolyte bath may be between about 0.5 and about 2.0. In accordancewith a preferred embodiment of the disclosure, the pH of the electrolytebath is between about 0.8 and about 1.2. The temperature of theelectrolyte bath may be between about 50° C. and about 90° C. Inaccordance with a preferred embodiment of the disclosure, thetemperature of the electrolyte bath may be between about 70° C. andabout 80° C. The electric current applied to the electrolyte bath mayhave a current density between about 0.2 Amps/in² to about 2.0 Amps/in².In accordance with one embodiment of the disclosure, the electriccurrent may have a current density between about 0.5 Amps/in² to about1.5 Amps/in².

In accordance with still another aspect of the disclosure, theconcentration of cobalt in the electrolyte bath may be between about 50grams per liter and about 200 grams per liter. In accordance with apreferred embodiment of the disclosure, the cobalt concentration in theelectrolyte bath may be about 100 grams per liter. The tribologicalparticles may be present in the electrolyte bath in a concentration fromabout 10 grams per liter to about 200 grams per liter. Specifically, thesilicon carbide tribological particles may be present in the electrolytebath in a concentration from about 10 grams per liter to about 200 gramsper liter. In accordance with a preferred embodiment of the disclosure,the silicon carbide tribological particles are present in theelectrolyte bath in a concentration from about 30 grams per liter toabout 60 grams per liter. By way of further example, the chromiumcarbide tribological particles may be present in the electrolyte bath ina concentration from about 10 grams per liter to about 200 grams perliter. In accordance with a preferred embodiment of the disclosure, thechromium carbide tribological particles are present in the electrolytebath in a concentration from about 35 grams per liter to about 100 gramsper liter. The tribological particles may have an average dimension, forexample, between about 0.1 micrometers and about 20 micrometers.

In accordance with still a further aspect of the disclosure, the articlemay be heat treated after the article has been coated to cause theprecipitation of cobalt-phosphides. The article may be heat treated at atemperature between about 150° C. and about 500° C. In accordance withone example, the article is heat treated at a temperature between about200° C. and about 400° C. The article may be heat treated for a lengthof time between about 15 minutes and about 180 minutes. The heattreatment temperature and duration are interrelated, in that a longerheat treatment may be appropriate at a lower temperature, and a shorterheat treatment may be appropriate at a higher temperature.

In further accordance with the disclosure, a system for electrolyticallycoating an article is provided comprising an electrolytic cell. The cellincludes an anode, a cathode capable of being placed in operablecommunication with an article to be coated, and an electrolyte bath. Theelectrolyte bath is in operable communication with the anode and thecathode. During electrolysis, the electrolyte comprises cobalt ions,phosphorous acid, and tribological particles selected from the groupconsisting of refractory materials, solid lubricants and mixturesthereof dispersed therein. The system also includes a direct currentpower supply adapted to apply steady direct current across the anode,electrolyte bath and cathode to coat an article with cobalt, phosphorousand the tribological particles. The system can include all of theattributes needed to carry out the method steps of the inventiondescribed herein.

In further accordance with the disclosure, a composition of matter isprovided. The composition of matter comprises cobalt, phosphorous andtribological particles selected from the group consisting of refractorymaterials, solid lubricants and mixtures thereof dispersed therein. Thecomposition of matter may be formed according to the processes describedherein. In accordance with one aspect of the disclosure, the coating mayhave a hardness of about 650-700 VHN. If the composition of matter isheat treated to form cobalt phosphides, the composition of matter may beharder. For example, the composition may include chromium carbidetribological particles and the coating may accordingly have a hardnessof about 500 VHN prior to heat treatment. In accordance with anotherembodiment of the disclosure the coating may include silicon carbidetribological particles and the coating may have a hardness of about 1150VHN subsequent to heat treatment.

In accordance with a further aspect, the disclosure provides a coatingfor improving the fatigue performance of an article. The coatingincludes a cobalt material matrix. The cobalt material matrix includes,and preferably consists of, a cobalt phosphorous alloy, wherein thephosphorous in the final coating is present in an amount between about 7weight percent and about 12 weight percent.

In accordance with one embodiment, the cobalt material matrix can besubstantially free of tribological particles. In this embodiment, thephosphorous in the final coating is preferably present in an amountbetween about 10 weight percent and about 12 weight percent. Inaccordance with another embodiment, the coating cab further include aplurality of tribological particles throughout the cobalt materialmatrix, the particles having an average particle size in the range offrom about 2 to 10 microns. In this embodiment, the phosphorous in thefinal coating is preferably present in an amount between about 7 weightpercent and about 8 weight percent.

In accordance with a further aspect, the coating preferably has anas-plated hardness of about 650-700 VHN and has a fatigue life that isgreater than an otherwise identical but uncoated article. Morepreferably, the article has a fatigue life that is at least twice asgreat as an otherwise identical but uncoated article. Even morepreferably, the article has a fatigue life that is at least three, four,five, six, seven, eight, nine or ten times as great as an otherwiseidentical but uncoated article.

In accordance with further aspects, the tribological particles includeceramic material selected from the group consisting of silicon carbide,chromium carbide, boron carbide, tungsten carbide, titanium carbide,silicon nitride, aluminum oxide, chromium oxide, and mixtures thereof.The tribological particles include solid lubricant material selectedfrom the group consisting of graphite, boron nitride, PTFE, molybdenumdisulfide, tungsten disulfide, and mixtures thereof

In accordance with another aspect, the disclosure provides a method forelectrolytically coating an article to enhance its fatigue performance.The method includes providing an article to be coated, and disposing thearticle in an electrolytic cell. The cell including a soluble anode, acathode in operable communication with the article, and an electrolytebath. The electrolyte bath, during electrolysis, includes cobalt ionsfrom the soluble anode and phosphorus obtained by separately introducingphosphorous acid into the bath, wherein the pH of the electrolyte bathis between about 1.2 and about 2.2. The method further includes applyingsteady direct electric current through the anode, the electrolyte bathand the cathode at a current density of about 0.3 Amps/in2 and about 0.8Amps/in2 to coat the article with a coating that is essentially free ofnickel and contains cobalt and phosphorous. The weight percent ofphosphorus in the resulting coating is between about 7% and about 12%.

In accordance with one embodiment, the weight percent of phosphorus inthe resulting coating is between about 7% and about 8%, wherein thecoating is substantially free of tribological particles. In accordancewith another embodiment, the electrolyte bath further includestribological particles selected from the group consisting of refractorymaterials, solid lubricants and mixtures thereof dispersed therein,wherein the resulting coating includes the tribological particles aftercoating the article, and further wherein the weight percent ofphosphorus in the resulting coating is between about 10% and about 12%.

In accordance with still a further aspect, the temperature of theelectrolyte bath is between about 70° C. and about 75° C. In oneembodiment, the phosphorous acid is introduced in a granular form.However, the phosphorus content of the bath and resulting article can beenhanced by introducing phosphoric acid into the bath as well.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and are intended toprovide further explanation of the invention claimed. The accompanyingfigures, which are incorporated in and constitute part of thisspecification, are included to illustrate and provide a furtherunderstanding of the methods, systems and articles of manufactureresulting from the invention. Together with the description, thedrawings serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an electroplating system made inaccordance with the present invention.

FIG. 2 is a photomicrograph showing the microstructure of a typicalCo—P—SiC electrocomposite coating containing about 5-6 weight percentphosphorous made in accordance with the present invention.

FIG. 3 is a photomicrograph showing the microstructure of a typicalCo—P—Cr₃C₂ electrocomposite coating containing about 5-6 weight percentphosphorous made in accordance with the present invention.

FIG. 4 is a schematic representation of a plating arrangement forplating articles in accordance with one aspect of the disclosure.

FIG. 5 is a chart describing the relationship between pH of a coatingbath as it relates to incorporation of phosphorus in the final coating.

FIG. 6 is a photomicrograph of a high fatigue cobalt phosphorus coatingincorporating tribological particles.

FIG. 7 is a chart describing the relationship of phosphorus content in acoating with as plated hardness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the disclosure, examples of which are illustrated in theaccompanying drawings. The methods and corresponding steps of thedisclosed embodiments will be described in conjunction with the detaileddescription of the compositions of matter and associated systems.

The devices and methods presented herein may be used for producingimproved coatings for articles that do not suffer from the deficienciesof coatings known in the prior art. The present invention may bepracticed using a generally conventional DC power supply to producecobalt-phosphorous base electrocomposite coatings having hardness, bendductility and corrosion resistance similar or superior to those of hardchrome. Unlike nickel and chromium, cobalt does not present significantenvironmental considerations when used in electroplating. As such, itpresents significant benefits over the use of techniques employingsignificant quantities of chromium or nickel.

In accordance with the invention, a system and associated method forelectrolytically coating an article is provided comprising anelectrolytic cell. The cell includes an anode, a cathode capable ofbeing placed in operable communication with an article to be coated, andan electrolyte bath. The electrolyte bath is in operable communicationwith the anode and the cathode. During electrolysis, the electrolytecomprises cobalt ions, phosphorous acid, and tribological particlesselected from the group consisting of refractory materials, solidlubricants and mixtures thereof dispersed therein. The system alsoincludes a direct current power supply adapted to apply steady directcurrent across the anode, electrolyte bath and cathode to coat anarticle with cobalt, phosphorous and the tribological particles.

For purpose of explanation and illustration, and not limitation, apartial view of an exemplary embodiment of the system in accordance withthe invention is shown in FIG. 1 and is designated generally byreference character 100. Other embodiments of a system in accordancewith the invention, or aspects thereof, are provided in FIGS. 2-3, aswill be described.

For purposes of illustration and not limitation, as embodied herein andas depicted in FIG. 1, system 100 is provided with a cell 110. Cell 110includes a container 112 adapted and configured to house an electrolytebath 114. Cell further includes an anode 116 and a cathode 126 inelectrical communication with a power supply 130.

The anode 116 may be formed from a variety of materials, for example,such as graphite, platinized copper, platinized titanium, platinizedcolumbium and combinations thereof. If desired, the anode 116 mayinclude a consumable portion (e.g., 118, 120) made from cobalt, whereinthe anode 116 is adapted to release cobalt ions into the electrolytebath 114 as cobalt is depleted from the bath, and deposited on anarticle to be coated. Suitable anodes 116 with consumable portions(e.g., 118 and/or 120) may be made in a variety of ways. For example,the anode 116 may be coated with cobalt to form a consumable portion 118of any desired geometry, such as by electroplating cobalt onto atitanium or stainless steel anode. Additionally or alternatively, pieces120 of cobalt may be disposed in a basket 122 or other suitablecontainer made at least in part, for example, from titanium or othersuitable conductive substantially non reactive material in communicationwith the electrolyte bath 114. The pieces 120 of cobalt dissolve when avoltage is applied across the anode 116 and cathode 126 to releasecobalt ions into the electrolyte bath 114. Specifically, electricalcurrent flows through the titanium basket 122 and to the cobalt, whichin turn oxidizes and goes into solution in bath 114. Pieces 120 ofcobalt metal are commercially available, for example, from AtlanticMetals and Alloys, Inc. in Stratford, CT. The source of cobalt ions mayadditionally or alternatively include an additional soluble cobaltsource selected, for example, from the group consisting of CoSO₄, CoCl₂,CoCO₃, Co(SO₃NH₂)₂ and mixtures thereof. Thus, for example, an inertanode 116 may be used, and additional CoSO₄ may be added to bath 114 toreplace cobalt in the bath as it is depleted due to deposition on thearticle to be coated and/or the cathode, as described in detail below.Suitable cobalt salts, such as cobalt sulfate, are commerciallyavailable, for example, from Shepherd Chemical Co., of Norwood Ohio, anddistributed, for example, by Gilbert and Jones Co., Inc., of NewBritain, Conn.

The cathode 126 may be made from a variety of materials as are known inthe art. In accordance with one embodiment of the invention, the cathode126 will generally include or otherwise be electrically attached to anarticle to be coated 128.

In accordance with a further aspect of the invention, an article may beelectroformed by coating cathode 126 with a coating material and thenreleasing the coating from the cathode 126. In accordance with thisaspect of the invention, the cathode 126 acts as a master, or mandrel,such that a “mirror” article is formed on the cathode by electroplatingmaterial onto the cathode 126. A variety of articles can be made in thismanner, such as leading edge blades for helicopters, complex, difficultto machine shapes such as small bellows, among others. Accordingly, inaccordance with this aspect of the invention, the cathode 126 can bemade from a material that does not adhere strongly to the coating, suchas passivated stainless steel. Stainless steel may be passivated by anyknown suitable method, for example, by exposure to hot chromic acid,nitric or citric acid to form an oxide layer on the cathode 126 torender it less reactive with a coating formed thereon.

It will be recognized that any suitable number of anodes 116 andcathodes 126 may be used, depending on what is being manufactured. Forexample, racks of articles 128 may be disposed in the electrolyte bath114 to be coated. Each article 128 is in conductive communication with,and effectively acts as a cathode 126. Any suitable number of solubleand/or inert anodes 116 can be used, as desired. It will also berecognized that the anode(s) 116 should be located suitably with respectto the cathode(s) 126. If it is desired to coat the interior of acylindrical article with a coating, it will be recognized that it issuitable to locate anode 116 within the cavity formed by the article.

The electrolyte bath 114 is in operable communication with the anode 116and the cathode 126. During electrolysis, the electrolyte bath 114comprises an electrolyte having cobalt ions, phosphorous acid andtribological particles selected from the group consisting of refractorymaterials, solid lubricants and mixtures thereof dispersed therein. Thecobalt ions can be introduced in a variety of ways, as described above.The concentration of cobalt in the electrolyte bath may be between about50 grams per liter and about 200 grams per liter, most preferably about100 grams per liter.

The electrolyte bath 114 may further comprise a dissolution promoter forpromoting the dissolution of the cobalt material. The dissolutionpromoter may include a halide salt. While a variety of salts can be usedas dissolution promoters, suitable dissolution promoters may include,for example, sodium chloride, cobalt chloride, bromide salts andcombinations thereof. In accordance with one embodiment, sodium chlorideis used as a dissolution promoter in electrolyte bath 114 in an amountof about 20 grams per liter.

The pH of the electrolyte bath 114 may be between about 0.5 and about2.0. In accordance with a preferred embodiment, the pH of theelectrolyte bath is between about 0.8 and about 1.2. During theelectroplating process, the pH of the electrolyte bath 114 increases. Inorder to maintain the pH within a desired range, one or more of avariety of buffering agents can be added to the electrolyte bath 114 tohelp maintain the pH within a desired tolerance. For example, a suitablebuffering agent is boric acid. If used, the boric acid can act to bufferbath 114, particularly in the region of the cathode 126, where hydroxidetends to form, since some hydrolysis can potentially occur at highcurrent densities. However, a buffering agent need not be used since thepH of bath 114 is generally very low, resulting in ample availablehydrogen ions in bath 114 that are available to readily combine with anyhydroxide formed by the cathode 126. If desired, pH adjustors may alsobe employed to increase or decrease the pH of the system. Suitable pHadjustors may include, for example, sulfuric acid, cobalt carbonate andsodium hydroxide. Cobalt carbonate is particularly attractive forincreasing the pH since it dissociates to form cobalt, which can be usedin plating, and carbon dioxide, which bubbles out of the bath 114 and isreleased to the atmosphere. It has been discovered that, while a varietyof factors affect the efficacy of the electroplating process embodiedherein, pH plays a significant role. As such, careful control of the pHof the electrolyte bath can lead to improved quality of the end-product.

It is also preferred to maintain a sufficient level of phosphorous acidin the electrolyte bath 114 suitable for electroplating a coating havingsufficient amounts of phosphorous. Preferably, the weight percent ofphosphorous in the resulting coating is between about 3% and 12%,preferably between about 4% and 7%. Accordingly, the phosphorous acidmay be present in the electrolyte bath in a concentration from about 3grams per liter to about 35 grams per liter. More preferably, thephosphorous acid is present in the electrolyte bath in a concentrationfrom about 3 grams per liter to about 25 grams per liter. Mostpreferably, the phosphorous acid is present in the electrolyte bath in aconcentration from about 3 grams per liter to about 15 grams per liter.If an inert anode 116 is used, the electroplating process is relativelyless efficient resulting in slower cobalt deposition on the cathode 126.In this example of an inert anode 116, a lower concentration ofphosphorous acid is needed. Specifically, since the reaction depositingcobalt is proceeding at a slower pace, relatively more phosphorous isdeposited for a given concentration of phosphorous acid. In contrast,when a soluble (e.g., consumable) anode 116 is used, the reaction todeposit cobalt is relatively more efficient. Accordingly, to obtainsuitable amounts of phosphorous in the coating, the concentration ofphosphorous acid is correspondingly increased.

For purposes of illustration and not limitation, as embodied herein,electrolyte bath 114 also includes tribological particles 102 dispersedtherein. The tribological particles 102 have superior tribologicalcharacteristics (i.e., characteristics that tend to cause a reduction infriction, an increase in lubrication and resulting decrease in the wearof surfaces containing the tribological particles 102) and preferablyinclude refractory materials and/or solid lubricants. These particlesare thus referred to as tribological particles herein. The refractorymaterials can include, for example, ceramics, diamond and mixturesthereof More specifically, ceramic tribological particles may beselected from the group consisting of silicon carbide, chromium carbide,boron carbide, tungsten carbide, titanium carbide, silicon nitride,aluminum oxide, chromium oxide, and mixtures thereof, among others.Solid lubricant tribological particles, such as graphite, boron nitride,PTFE, molybdenum disulfide, tungsten disulfide, and mixtures thereof mayalso be used. It will be recognized that certain tribological particles,such as boron nitride, have both ceramic and lubricious properties.

The tribological particles 102 in the electrolyte bath 114 may have anaverage dimension, for example, between about 0.1 micrometers and about20 micrometers. In accordance with a preferred embodiment of theinvention, the tribological particles have an average dimension betweenabout 1.0 micrometers and about 5.0 micrometers. If silicon carbidetribological particles are employed, they may be present in theelectrolyte bath in a concentration from about 10 grams per liter toabout 200 grams per liter, preferably from about 30 grams per liter toabout 60 grams per liter. If chromium carbide tribological particles areused, they may be present in the electrolyte bath in a concentrationfrom about 10 grams per liter to about 200 grams per liter. Inaccordance with a preferred embodiment of the invention, the chromiumcarbide tribological particles are present in the electrolyte bath in aconcentration from about 35 grams per liter to about 100 grams perliter.

FIG. 2 is a cross-sectional photomicrograph of a coating showing themicrostructure of a typical Co—P—SiC electrocomposite coating containingabout 5-6 weight percent phosphorous. Similarly, FIG. 3 is across-sectional photomicrograph of a coating showing the microstructureof a typical Co—P—Cr₃C₂ electrocomposite coating containing about 5-6weight percent phosphorous. The tribological particles occupy about 25%of the volume of each of the coatings depicted in FIG. 2 and FIG. 1 Thesamples depicted in FIGS. 2 and 3 have not been heat treated. As can beseen in the Figures, the tribological particles 102 are dispersedthroughout the coating 200. As further depicted, the coating 200 ismetallurgically sound and crack-free. In contrast, a chromium coatinggenerally demonstrates many micro cracks throughout the coating whichdegrade its corrosion resistance.

The temperature of the electrolyte bath 114 may be between about 50° C.and about 90° C. Temperatures below about 50° C., while possible, can bedisadvantageous because of lower deposition rates of the coating andinefficient incorporation of phosphorous into the coating. On the otherhand, temperatures in excess of about 90° C. generally results inexcessive loss of material from the electrolyte bath 114 by way ofevaporative mechanisms. In accordance with a preferred embodiment of theinvention, the temperature of the electrolyte bath may be between about70° C. and about 80° C.

As depicted in FIG. 1, direct current power supply 130 is adapted toapply steady direct current across the anode 116, electrolyte bath 114and cathode 126 to coat an article (e.g., 128) with cobalt, phosphorousand the tribological particles. In operation, the electric currentapplied to the electrolyte bath may have a current density between about0.2 Amps/in² to about 2.0 Amps/in². In accordance a preferred embodimentof the invention, the electric current may have a current densitybetween about 0.5 Amps/in² to about 1.5 Amps/in². Power supply 130 canbe similar to rectifiers as are known in the art, such as ModelP-106-.25CF rectifier commercially available from Aldonex, Inc. inBellwood, Ill., among others.

Prior art, such as U.S. Pat. No. 5,352,255 to Erb et al. describe nanocrystalline cobalt phosphorous coatings with a grain size smaller than100 nm. Such coatings have characteristics either similar or superior tohard chrome and can be used as a replacement of hard chrome. However, toform nanocrystalline cobalt phosphorous coatings, it is necessary to usecomplex and expensive pulsed DC power supplies. Applicants havediscovered that the addition of tribological particles 102 as describedherein to the electrolyte bath has made it possible to produce ametallurgically sound, crack free coating with high hardness andductility which can be used to replace hard chrome. Unlike the teachingsof Erb et al., the systems made in accordance with the invention arecapable of using the conventional steady DC power supplies known in theart.

In accordance with still a further aspect of the invention, the coatingformed on the article coated during the electroplating process may beheat treated to cause the precipitation of cobalt-phosphides within thecoating. To cause this precipitation, the article may be heat treated inan oven, for example, in the presence of air. Suitable ovens can beobtained from Lindberg/Blue of Thermo Electron Corp. located inAsheville, N.C. A Lindberg furnace Type No. 51662 was used to performthe heat treatments described in the Examples below, but it will berecognized that other similar furnaces are suitable.

The heat treatment can occur, for example, at a temperature betweenabout 150° C. and about 500° C. for a length of time between about 15minutes and about 180 minutes. In accordance with one embodiment, thearticle is heat treated at a temperature between about 200° C. and about400° C. The heat treatment temperature and duration are interrelated, inthat a longer heat treatment may be appropriate at a lower temperature,and a shorter heat treatment may be appropriate at a higher temperature.

In further accordance with the invention, a composition of matter isprovided comprising cobalt, phosphorous and tribological particlesselected from the group consisting of refractory materials, solidlubricants and mixtures thereof dispersed therein. The composition ofmatter may be used as a protective coating applied to an article, or mayconstitute a separate member electroformed on a mandrel as describedherein. The composition of matter may be formed, for example, accordingto the processes described herein.

Prior to heat treatment, the cobalt-phosphorous-tribological particlecoating generally has a hardness of about 650-700 VHN. If this coatingis heat treated to precipitate cobalt phosphides, the resulting coatingis harder. Experience has resulted in coatings comprising cobalt,phosphorous and chromium carbide tribological particles having ahardness of about 1000 VHN or greater. Coatings using silicon carbideinstead of chromium carbide have been formed having a hardness of about1150 VHN or greater. The desired characteristics of coatings disclosedherein are maintained by controlling electroplating parameters andelectrolyte bath composition as described herein.

The following Examples further illustrate the present invention. Unlessotherwise indicated, stated percentages are by weight.

Example I

Carbon steel samples were plated in accordance with the presentinvention. An electroplating bath was provided having the followingcomposition:

-   -   Cobalt sulfate: 520 g/l    -   Boric acid: 40 g/l    -   Sodium chloride: 20 g/l    -   Granular phosphorous acid: 15 g/l    -   Silicon carbide particles(2-5 microns): 25 g/l

The bath was made by mixing the above ingredients in water to a totalvolume of 3.5 liters. Electroplating was performed with cobalt pieces ina titanium basket used as an anode and plain carbon steel panels ascathode. One side of each carbon steel panel was masked and the sidefacing the anode was plated with a cobalt-phosphorous-silicon carbidecoating.

Plating Conditions

The bath pH was maintained at about 0.9 with sulfuric acid to lower pHand sodium hydroxide to raise pH. The bath temperature was maintainedbetween about 70° C.-80° C. The samples were plated at a current densityof 2 Amperes/square inch. The panels were plated for about an hour whichproduced a coating thickness around 0.005 inch.

Coating Properties

Phosphorous content of the coating was about 9 wt %. As-plated hardnessof the coating was 720 VHN. The coating was heat treated in air at 400°C. for 1.5 hrs. The as heat treated hardness was 1150 VHN.

Example II Comparison with Hard Chrome

Materials made in accordance with the invention have properties equalingor even exceeding those of hard chrome as shown in Table I, below. TableI compares conventional hard chrome processing with exemplary parametersprovided by the present invention. As can be seen, materials made inaccordance with the present invention compare favorably with chrome andsignificantly surpass chrome in corrosion prevention.

TABLE I Comparison of Co—P—SiC and Hard Chrome Feature Co—P—SiC HardChrome Power supply Conventional DC Conventional DC Plating rate Up to0.005″/hr Up to 0.0016″/hr Thickness Plated up to 0.02″ Typically <0.02″As-plated condition Crack free Micro cracked Micro structure ~50 nmgrains Normal grain size, with 2-5 μm SiC >1000 nm particles As-platedhardness 650 800-1200 As heat treated hardness, 760 — 200° C./.1.5 hrsAs heat treated hardness, 1200 — 400° C./1.5 hrs Bend ductility, 0.003″A few fine No visible cracks at the thick, 90° bend cracks at the bendbend Threshold strain* Similar to HVOF Much lower than T-400** coatingHVOF T-400 coating Corrosion resistance No visible rust Rust after 24hrs Salt fog test (ASTM B117) even after 200 hrs *Total strain toinitiate a crack. **T-400 is tribaloy 400 coating deposited by usingHVOF thermal spray process

Example III Effect of Phosphorous Acid Concentration on Hardness

It has also been discovered that the amount of phosphorous acid in theelectrolyte bath has a measurable effect on the hardness of the producedcoating. For example, lowering the concentration significantly below 5grams per liter or raising it significantly above 25 grams per literbegins to show a drop off in coating hardness, as shown in Table II andTable III, below.

TABLE II As-plated and as-heat treated hardness of Co—P—SiC* coatings asfunction of H₃PO₃ in the plating electrolyte bath. H₃PO₃ As-heat treatedhardness. concentration As-plated hardness HT @ 400° C. for 1.5 hours  0g/L 360 VHN  350 VHN  5 g/L 669 VHN 1012 VHN 15 g/L 720 VHN 1147 VHN 25g/L 736 VHN 1236 VHN 35 g/L 660 VHN 1150 VHN *Concentration of SiC is 25g/L in plating bath.

TABLE III As-plated and as-heat treated hardness of Co—P—Cr₃C₂* coatingsas a function of H₃PO₃ in the plating bath. H₃PO₃ As-heat treatedhardness. concentration As-plated hardness HT @ 400° C. for 1.5 hours  0g/L 360 VHN  350 VHN  9 g/L 663 VHN 1008 VHN 15 g/L 670 VHN 1053 VHN 25g/L 681 VHN 1089 VHN 35 g/L 636 VHN 1019 VHN *Concentration of Cr₃C₂ is50 g/L in plating bath.

Example IV Increase in Hardness by Adding Tribological Particles

Table IV compares the as plated and as heat treated hardness ofcobalt-phosphorous with composite cobalt-phosphorous coatings furtherincluding chromium carbide and silicon carbide. Tables V and VI belowshow the relative increase in hardness of the cobalt-phosphorous coatingwith the composite coatings. As can be seen, the addition of the carbidetribological particles results in a surprising increase in the hardnessof the material after the precipitation of cobalt-phosphides.

TABLE IV As-plated and as-heat treated hardness of Co—P, Co—P—Cr₃C₂ andCo—P—SiC coatings with 5 g/L H₃PO₃ in the plating bath. Samples wereheat treated at 325° C. for 0.5 hours. As-plated As-heat treatedhardness hardness HT @ 325° C. for Hardness Coating (VHN) 0.5 hoursincrease Co—P 650  700 VHN  50 VHN Co—P—Cr₃C₂* 670 1010 VHN 340 VHNCo—P—SiC** 669 1150 VHN 480 VHN *50 g/L Cr₃C₂ in plating bath **25 g/LSiC in plating bath

TABLE V As-plated and as-heat treated hardness of Co—P and Co—P—SiC with5 g/L H₃PO₃ in the plating bath. Samples were heat treated at 205° C.and 400° C. for 1.5 hours. As-heat treated As-heat treated hardnesshardness HT @ 205° C. HT @ 400° C. for 1.5 for 1.5 Coating As-platedhardness hours hours Co—P 650 VHN 688 VHN 1000 VHN (Δ = 38 VHN) (Δ = 350VHN) Co—P—SiC** 669 VHN 756 VHN 1216 VHN (Δ = 87 VHN) (Δ = 547 VHN) **25g/L SiC in plating bath

Example V Comparative Enhanced Bend Ductility

The Co—P—SiC and Co—P—Cr₃C₂ coatings also have superior bend ductilitycompared to the Co—P coating having similar wt % P and coatingthickness. For example, steel panels 4″×1″×0.04″, were plated with about0.002″ coatings using coating conditions described herein. Panels werecoated on one side only by masking the other side. The panels were heldin a vice and bent through 180° in the middle of the panels with thecoating on the convex side of the bend. The coating was examined forcracks and delamination. The majority of the panels coated only withcobalt and phosphorous (i.e., without tribological particles) showedlarge cracks or complete delamination at the bent convex surface.

In surprising contrast, the Co—P—SiC and Co—P—Cr₃C₂ coatings did notdelaminate. To the contrary, only fine cracks were observed at the bend.This simple bend test, although qualitative, does indicate an enhancedductility of the Co—P—SiC and Co—P—Cr₃C₂ coatings. Generally, it wouldbe expected that inclusion of tribological particles would make thecoating more brittle. However, the Co—P—SiC and Co—P—Cr₃C₂ coatingspossess an unexpected combination of high hardness and ductility. It hasgenerally been discovered that the heat treatment temperatures toemphasize ductility are lower than those used to increase hardness.

Some of the deficiencies in the prior art vis-à-vis the fatigue debitthat accompanies plating articles are set forth in the backgroundsection above. It is thus appreciated that a wear/corrosion resistantplated coating that does not induce a fatigue debit to the ferrousmaterial substrate (or other substrate) would be advantageous. Hardwarecross sections could be reduced, thus reducing weight and size ofcomponents since the plated coating would not impart a fatigue debit. Inanother aspect, the present disclosure provides electroplated Co—P andCo—P—SiC coatings that provide corrosion and wear resistance withoutaffecting fatigue characteristics or inflicting a “fatigue debit”. Thesecoatings are generally fully dense without any inherent micro-cracksthat form the nucleation sites of fatigue cracks. These coatings can beelectroplated using conventional DC power and therefore they are drop-inreplacements for hard chrome without the adverse environmental impact ofhard chrome.

In accordance with one aspect, a high strength steel AISI 4130, commonlyused for engineered applications is selected as the base material todemonstrate the unique no “fatigue debit” characteristics ofelectroplated Co—P and Co—P—SiC. It is believed that similar effects canbe demonstrated for other materials such as superalloys, titaniumalloys, aluminum alloys and the like.

Example VI

Fatigue strength was determined using the four-point rotating beam testmethod, ISO 1143. Appropriate round bar samples with a 0.250″ minimumdiameter at the center were coated with Co—P—SiC. The composition of theplating bath can be found in Table 6. The fatigue bars were heat treatedto a hardness of 32-34 HRC, stress relieved at 375° F. for 3 hours minand polished to a surface finish of 2-4 μinch Ra. Coating thickness wasabout 0.002″. After coating, the samples were again polished to a 2-4μinch Ra surface finish. Electroplating conditions were as follows:

TABLE 6 Electroplating bath composition Component Concentration Cobaltsulfate 500 g/L  Phosphorus acid 12 g/L Silicon carbide particle, 2-5 um50 g/L Boric acid 35 g/L Sodium chloride 20 g/L Electroplating processparameters: pH = 1.25; current density = 0.3 Amperes/square inch;voltage = 3.5-4.0 v; temperature = 72-78° C.

Samples were plated with soluble cobalt anodes. Samples (i.e., thecathodes) were attached to a rotating fixture, as shown in FIG. 4 whichmaintained electrical contact during rotation. This rotating arrangementwas expected to provide uniform coating microstructure all throughoutthe gage length of the test bars. To determine effects of coating onfatigue strength, uncoated bars were also tested. The results aresummarized in Table 7:

TABLE 7 Cycle Life of Co—P—SiC Electroplated and Uncoated 4130 SteelRotating Beam Fatigue Bars with Maximum Bending Stress of 100,000 psiAverage of Fatigue Testing Bars Fatigue Cycles Fatigue Cycles Uncoated4130 bar 1 187,289 Uncoated bar: 156,252 Uncoated 4130 bar 2 130,432Uncoated 4130 bar 3 151,035 Co—P—SiC coated 4340 bar 1 638,757 Coatedbar: 884,903 Co—P—SiC coated 4340 bar 2 1,065,222 Co—P—SiC coated 4340bar 3 744,179 Co—P—SiC coated 4340 bar 4 1,091,453

It is evident that the Co13 P13 SiC coated samples had a significantlysuperior average fatigue life compared to that of uncoated samples.Unlike hard chrome, the novel Co—P—SiC coating plated with above processparameters demonstrated no “fatigue debit” for the base material.

Example VII

Rotating bar samples, coated with Co—P, were prepared the same way asthe Co—P—SiC coated samples. The solution composition used can be foundin Table 8. Electroplating process parameters: pH=1.3-1.4; currentdensity=0.7-0.8 amps/square inch; voltage=3.5-4.0v; temperature=72-78°C. After the samples were plated they were subjected to a 10 hr 375° F.hydrogen embrittlement relief bake. Table 9 summarizes the results.

TABLE 8 Electroplating bath composition Component Concentration Cobaltsulfate 450 g/L  Phosphorus acid 30 g/L Boric acid 35 g/L Sodiumchloride 20 g/L

TABLE 9 Fatigue Life of Co—P Electroplated and Uncoated 4130 SteelRotating Beam Fatigue Bars with Maximum Bending Stress of 100 KSIAverage of Fatigue Testing Bars Fatigue Cycles Fatigue Cycles Uncoated4130 bar, #205 187,289 156,252 #206 130,432 #207 151,035 Co—P coated4130 bar, #250 488,763 665,763 #251 842,762

Example VIII

Rotating bar samples, coated with Co—P, were prepared in the same way asdescribed in Example VII, but the bars were made of 15-5 PH stainlesssteel. All the samples were exposed to a hydrogen embrittlement reliefbake for 10 hrs at 375° F. after plating. The bars were run at 90, 100,110 and 120 KSI stress levels and the results are tabulated in Table 10.As is evident with all the stress levels, no fatigue debit was observedfor the Co—P plated samples.

TABLE 10 Fatigue Life of Co—P Electroplated and Uncoated 15-5 PHStainless Steel Rotating Beam Fatigue Bars with Maximum Bending Stressof 90, 100, 110 and 120 KSI Fatigue Fatigue Fatigue Fatigue Cycles, 90Cycles, 100 Cycles, 110 Cycles, 120 Bar # KSI Bar # KSI Bar # KSI Bar #KSI Uncoated Fatigue Testing Bars #1 674,107 #7 159,665 #2 163,969 #390,570 #5  753760 #8 275,601 #11 156,989 #14 72,917 #6  769961 #9309,426 #12 221,508 #15 55,015 #4 1,035,103*  #10 401,332 #13 126,064Avg 808,233 286,506 167,133 72,834 cycles Co—P Coated Fatigue TestingBars #16 1,005,065*  #20  850,145** #24 1,053,171*  #28 398,642 #171,012,889*  #21 1,005,151*  #25 1,067,568*  #29 328,037 #18 551,285 #221,004,892*  #26 797,223 #30 180,141 #19 1,047,888*  #23 211,057 #27 787,687** Avg 904,281 767,811 926,412 302,273 cycles *Bar was removedfrom test and did not break **Bar was removed from test because thecollet slipped off the shank, bar did not break

Applicants believe that the extraordinary benefits described hereinvis-à-vis enhanced fatigue life correlate, in one aspect, with theweight percent of phosphorus in the resulting coating. Applicants haveexperimentally arrived at a curve illustrated in FIG. 5 that provides ameans for controlling the weight percent of phosphorus in the resultingcoating as a function of pH. It will be noted that this curve correlatesdirectly with a Co—P coating. It is believed that similar behavior isencountered when incorporating tribological particles in the matrix.However, it is believed that the uptake of phosphorus in the coating isgenerally enhanced by the presence of such particles, such as particlesof silicon carbide.

FIG. 5 illustrates Co13 P bath pH vs. wt % P in the resulting coating atthree levels of phosphorus acid, 10 g/l, 20 g/l and 30 g/l in the bath.The cobalt content was maintained at about 105-115 g/l, current densityat 0.5 ASI, temperature at 70-75° C., and plating voltage at 9-11 v(rotating fixture and single anode piece). Coating wt % P linearlydecreased as a function of pH. The phosphorous acid decreased atapproximately 0.5 g/l after plating each panels for about 8-9 amp.hr andabout 1.3 g of phosphorous acid is added after plating each panel.Cobalt increased after plating each 4″×1″ panels by about 2 g/l for 2.5l bath after 8-9 amp.hr of plating. Plating baths were adjusted afterplating 5-6 samples to maintain cobalt within a 105-115 g/l range. Bathswere also analyzed frequently. Coatings were peeled off from 4″×1″panels to analyze for % P.

Applicants have discovered that the optimum range of weight percent ofphosphorus for high fatigue components is substantial, particularly fromabout 7 wt % to about 12 wt %, more preferably about 7 wt % to about 11wt %. Correspondingly, the pH range is similarly quite wide, from about1.2 to about 2.2 based on phosphorus acid concentration at a specificcurrent density of 0.5 amperes per square inch and within a temperaturerange of 70-75° C.

Moreover, it has been discovered that long cycle life approaching 10× ofan uncoated part, was obtained for both Co—P and Co—P—SiC coatings,although Co—P—SiC coatings had more long life samples. The compositionand plating conditions that exhibited more frequent long cycle life(5×-10×) involves a bath having about 110 g/l of cobalt and 20 g/l ofphosphorus acid at plating conditions of a current density of 0.5amperes per square inch at a pH of about 1.8. For such a coating, basedon FIG. 5, it is estimated that this coating has a weight percent ofphosphorus of about 7.5 percent. For Co—P base coatings without SiC, thecorresponding percent phosphorus uptake is about 11 weight percentphosphorus. FIG. 6 presents an exemplary photomicrograph of ananocrystalline Co—P matrix containing 2-5 micron diameter SiCparticles. No cracks or pits are present, which are typically found inhard chrome. Thus, it is believed that using coatings as describedherein helps extend fatigue live by providing a coating that is free oressentially free of imperfections that can act as crack initiationsites. It will be appreciated that the other tribological particlesdescribed earlier in this disclosure besides SiC can be incorporatedindividually or in combination.

Applicants believe that the micro-structure of the subject coatings aretransition from nano-crystalline to nano-crystalline/amorphous hybrid,to amorphous substantially as set forth in FIG. 7. FIG. 7 illustratesthe increase in hardness with the increased incorporation of phosphorusin the coating, wherein the coating is as coated, before heat treatment.Hardness is improved with heat treatment of the Co—P matrix byprecipitation of cobalt-phosphide. It is believed that the tribologicalparticles add a synergistic effect and further improve hardness comparedto Co—P matrix alone. Heat treatment temperatures can be selected tomaximize hardness or toughness (lower temperatures for toughness, highertemperatures for hardness).

The compositions of matter, methods and systems of the presentinvention, as described above and shown in the drawings, provide for amaterial with superior properties including enhanced corrosionresistance, and hardness and other properties similar to hard chrome,without the environmental hazards associated with electroplatingchromium, but also including enhanced fatigue resistance. It will beapparent to those skilled in the art that various modifications andvariations can be made in the present invention without departing fromthe spirit or scope of the invention. Thus, it is intended that thepresent invention include modifications and variations that are withinthe scope of the appended claims and their equivalents.

1. A coating for improving the fatigue performance of an article, saidcoating comprising: a) a cobalt material matrix; b) said cobalt materialmatrix consisting of a cobalt phosphorous alloy, wherein the phosphorousin the final coating is present in an amount between about 7 weightpercent and about 12 weight percent.
 2. The coating of claim 1, whereinthe cobalt material matrix is substantially free of tribologicalparticles, and wherein the phosphorous in the final coating is presentin an amount between about 10 weight percent and about 12 weightpercent.
 3. The coating of claim 1, wherein the coating further includesa plurality of tribological particles throughout the cobalt materialmatrix, the tribological particles having an average particle size inthe range of from about 2 to 10 microns, and wherein the phosphorous inthe final coating is present in an amount between about 7 weight percentand about 8 weight percent.
 4. The coating of claim 1, wherein thecoating has an as-plated hardness of about 650-700 VHN and has a fatiguelife that is greater than an otherwise identical but uncoated article.5. The coating of claim 1, wherein the article has a fatigue life thatis at least twice as great as an otherwise identical but uncoatedarticle.
 6. The coating of claim 1, wherein the article has a fatiguelife that is at least three times as great as an otherwise identical butuncoated article.
 7. The coating of claim 3, wherein the tribologicalparticles include ceramic material selected from the group consisting ofsilicon carbide, chromium carbide, boron carbide, tungsten carbide,titanium carbide, silicon nitride, aluminum oxide, chromium oxide, andmixtures thereof.
 8. The coating of claim 3, wherein the tribologicalparticles include solid lubricant material selected from the groupconsisting of graphite, boron nitride, PTFE, molybdenum disulfide,tungsten disulfide, and mixtures thereof.
 9. A method forelectrolytically coating an article to enhance its fatigue performance,comprising: a) providing an article to be coated; b) disposing thearticle in an electrolytic cell, the cell including a soluble anode, acathode in operable communication with the article, and an electrolytebath, the electrolyte bath, during electrolysis, comprising cobalt ionsfrom the soluble anode, phosphorus obtained by separately introducingphosphorous acid into the bath, wherein the pH of the electrolyte bathis between about 1.2 and about 2.2; and c) applying steady directelectric current through the anode, the electrolyte bath and the cathodeat a current density of about 0.3 Amps/in² and about 0.8 Amps/in² tocoat the article with a coating that is essentially free of nickel andcontains cobalt and phosphorous, wherein the weight percent ofphosphorus in the resulting coating is between about 7% and about 12%.10. The method of claim 9, wherein the weight percent of phosphorus inthe resulting coating is between about 7% and about 8%, and furtherwherein the coating is substantially free of tribological particles. 11.The method of claim 9, wherein the electrolyte bath further includestribological particles selected from the group consisting of refractorymaterials, solid lubricants and mixtures thereof dispersed therein,wherein the resulting coating includes the tribological particles aftercoating the article, and further wherein the weight percent ofphosphorus in the resulting coating is between about 10% and about 12%.12. The method of claim 9, wherein the temperature of the electrolytebath is between about 70° C. and about 75° C.
 13. The method of claim 9,wherein the phosphorous acid is introduced in a granular form.
 14. Themethod of claim 9, wherein the resulting coated article has an as-platedhardness of about 650-700 VHN and has a fatigue life that is greaterthan an otherwise identical but uncoated article.
 15. The method ofclaim 9, wherein the resulting coated article has a fatigue life that isat least twice as great as an otherwise identical but uncoated article.16. The method of claim 9, wherein the resulting coated article has afatigue life that is at least three times as great as an otherwiseidentical but uncoated article.